Digital communication systems comprise time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and WCDMA telecommunication standards. Digital communication systems also include cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system. Another example of digital communication system is Orthogonal Frequency Division Multiplex (OFDM) based networks.
The technology described in this application will be described in the context of a WCDMA based UMTS network, but it should be noted that it is not limited to such a network. The radio access network of a UMTS network is referred to as UMTS terrestrial radio access network (UTRAN). The UTRAN is illustrated in FIG. 1 and comprises at least one Radio Network System (RNS) connected to the Core Network (CN). The CN is connectable to other networks such as the Internet, other mobile networks e.g. GSM systems and fixed telephony networks. The RNS comprises at least one Radio Network Controller (RNC). Furthermore, the respective RNC controls a plurality of Node Bs, also referred to as base stations, that are connected to the RNC. Each Node B covers one or more cells and is arranged to serve the User Equipment (UE) within said cell. Finally, the UE, also referred to as mobile terminal, is connected to one or more Node Bs over the Wideband Code Division Multiple Access (WCDMA) based radio interface.
This specification focuses on WCDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in other digital communication systems in which an identity is used to identify a user or a channel in the uplink direction. The identity used in the WCDMA network described above will be described below.
WCDMA based UMTS network is based on direct-sequence spread-spectrum techniques. Two different spreading codes are used for identifying users (i.e. the mobile terminals) and physical channels in the uplink direction (i.e. from the mobile terminal-to-the base station). The spreading codes are added in two steps, first channelization codes with repeated cycles of symbol length, and then scrambling codes with cycles much longer than the symbol lengths.
In the uplink, the scrambling codes are used to identify the users, and the channelization codes are used to identify the different dedicated channels for a specific user. E.g. in concurrent voice and data communication within a mobile terminal, channelization codes are used to identify the respective dedicated channels. This structure is also used by e.g. CDMA2000 networks which is another example of a CDMA based network.
Since the users share the same radio resource in CDMA systems, it is important that each physical channel does not use more power than necessary. This is achieved by power control mechanism, in which the base station and/or RNC estimates the signal-to-interference ratio (SIR) for its dedicated physical channel (DPCH), compares the estimated SIR against a reference value, and informs the mobile terminal to adjust the mobile station's transmitted DPCH power to an appropriate level. WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology. Scrambling and channelization codes and transmit power control are well known in the art.
Uplink interference mitigation is seen as an important method that may increase the capacity and/or coverage of a cellular system. One interference mitigation method is interference cancellation. Since the perceived uplink interference contribution from each link is reduced with interference cancellation, each mobile can use a lower transmission power to reach an acceptable signal to interference and noise level after some of the interference has been cancelled. Coverage probability is directly related to the link budget, or more precisely the probability that the required transmission power is lower than the maximum transmission uplink power. A lower interference level with interference cancellation means that the available uplink power will suffice further away from the base station. Similarly, the lower interference from other users also means that the interference cancellation gain can be seen as a capacity gain, allowing more users to operate. Uplink capacity is related to uplink noise rise, i.e. total received uplink power relative the noise power. Since the perceived uplink interference contribution from each link is reduced, more users can be admitted to meet the uplink noise rise requirement when using interference cancellation. Hence, the interference cancellation gain can either be utilized as a coverage gain or a capacity gain, or a combination of both.
Another interference mitigation method is multi-user detection. Typically, the multi-user receiver utilizes channel estimates related to connected users to improve decoding performance over a conventional RAKE receiver. The optimal solution is typically not tractable, but extensive research has been focused on sub-optimal receivers. They all aim at estimating the channel from known, i.e. connected, users. However, users connected to other cells are seen as non-cancelable interference.
In 3GPP Release 6, the WCDMA standard is extended with the Enhanced Uplink concept. This concept introduces the Enhanced Dedicated Transport Channel, E-DCH. A further description can be found in 3GPP TS 25.309 “FDD Enhanced Uplink; Overall description”. The Enhanced Uplink concept allows considerably higher peak data-rates in the WCDMA uplink.
With the above enhanced uplink concept, it is possible to face the situation where one or a few users are allocated all the uplink resources. In such a situation the possible inter-cell interference contribution from one single user can be very large. The control over these users is restricted to the serving Node B (typically the best serving Node B in the downlink), which have good and accurate control via the absolute grant channel and the control is also restricted to the other Node Bs in the active set, which have less accurate control via the relative grant channel. I.e. the serving Node B is able to control the data rate, and thus the interference, of E-DCH UEs very detailed by means of the absolute grant channel (AGCH) while the other Node Bs in the E-DCH active set (the E-DCH active set comprises the base stations having an E-DCH connection to the UE) may also influence the data rate, but not that detailed, e.g. only relatively, via the relative grant channel (RGCH). Node Bs outside the E-DCH active set are however not able to control the data rate at all which implies that the Node Bs outside the E-DCH active set cannot control the inter-cell interference.
It should be noted that this problem is not limited to a WCDMA network with enhanced uplink, but the possible inter-cell interference contribution from one single user can be much larger for E-DCH users. Such high inter-cell interference critically limits the E-DCH peak data rates. In this case interference cancellation is not an option, since the corresponding scrambling codes are not known outside the E-DCH active set.
Inter-cell interference in other cellular networks such as GSM/EDGE, D-AMPS IS-136, IS-95, cdma2000, evolved 3G, 4G, OFDM systems etc also occurs since each base station is only able to control the allocation of resources within its service area. Such inter-cell interference may degrade the system performance and reduce the system capacity.